196 



EVOLUTION, GENETICS, AND EUGENICS 



ground of embryological lore. In very brief, it must suffice for present 

 purposes to state that differentiation is accomplished partially by un- 

 equal cell division, in which the two daughter cells come to possess 

 different cytoplasmic contents; partially by the differences in position 

 of cells with respect to the environment; partially by differences in 



the relations of certain cells to 

 other cells that have already as- 

 sumed special characters owing to 

 their original position or their spe- 

 cial relation to the environment; 

 and partially by the fact that cer- 

 tain genes are effective only at a 

 given time in development or when 

 cells are in a certain physiological 

 state. All cells in an organism are 

 believed to possess the same genes, 

 but particular genes become effec- 

 tive only under special conditions 

 of time and place. Some such 

 complex of intricate interrelations, 

 at present very obscure even to the 

 most advanced students of embry- 

 ology, seems to underly cellular 

 differentiation. While the geneti- 

 cist cannot avoid all responsibility 

 for the solution of these insistent 

 problems, he may with some ap- 

 propriateness delegate them, for 

 the present at least, to the experi- 

 mental embryologist. At any rate, 

 it seems best at the present juncture to sidestep the problems of dif- 

 ferentiation and to get back on surer ground, more familiar to geneti- 

 cists and more applicable to their problems. 



Fig. 43. — Diagram of the end 

 phases of mitosis. //, the beginning of 

 constriction of the cell membrane; I, 

 the completed division. {From Wilson.) 



THE ORIGIN OF NEW INDIVIDUALS 



MODES OF REPRODUCTION 



Since individuals, the temporary components of the race or species, 

 are continually running their courses and dying off, replacements are 

 necessary if the race is to persist. The replacement of old individuals 

 by new is commonly called reproduction. There are numerous modes 



